EP2862614B2 - Filter system with filter element - Google Patents

Description

technical field

The invention relates to a filter system with a filter element, in particular for use as an air filter of an internal combustion engine, and a filter element for installation in such a filter system.

State of the art

The structure of filter systems in which round filter elements are used is known from the prior art. In use, the round filter elements are frequently flowed through from the outside in, with the filtered fluid being guided from the cylindrical interior through an opening in the cover or bottom of the round filter element to the outlet of the filter housing. The diameter of the outlet opening in the cover of the round filter element is limited by the diameter of the interior of the filter element. This results from the diameter of the filter element minus the fold height of the filter medium used.

On the one hand, the largest possible fold height allows a large effective filter surface of the filter element with the same external dimensions. On the other hand, the outlet opening for the filtered fluid must have a sufficient cross-section to be able to drain the fluid. Given the specified dimensions of the filter element, these two design influences must therefore be weighed up and a compromise found.

From the DE 198 49 998 A1 a filter cartridge is known which is cylindrical on the outside and has a conical interior.

From the WO 2014/085769 A2 a secondary filter element having an unpleated end and a pleated end is known.

From the EP 1558360 B1 Conically shaped filter elements are known, which, however, have a larger installation space requirement due to the conical shape, which opens at a certain angle counter to the outlet direction of the filter system. Folded secondary elements are also known, but they take up additional space compared to secondary elements with smooth filter media.

It is therefore an object of the invention to design a compact filter system with at least one replaceable filter element made of a filter medium in such a way that the highest possible separation rate for dust particles can be achieved with a favorable installation space.

A further object of the invention is to create a filter element for installation in such a filter system, with which the highest possible separation rate for dust particles can be achieved with a favorable installation space.

The aforementioned objects are achieved with a filter system and a use according to one of the independent claims.

Favorable configurations and advantages of the invention result from the further claims, the description and the drawing.

Disclosure of Invention

A filter system with one or more filter elements equipped with at least one filter medium is proposed, with at least one filter element extending along a longitudinal axis, with the filter system comprising a housing with a housing wall and a cover, with an inlet on the housing for supplying a fluid to be filtered , in particular air, and an outlet for discharging the filtered fluid is arranged, wherein the filter medium is folded in a zigzag shape and has a plurality of outer edges extending along the longitudinal axis and the outer edges on the opposite side of the filter medium opposite inner edges of folds, and wherein the filter medium has a variable flow resistance along the longitudinal axis. The variable flow resistance along the longitudinal axis causes a particularly effective separation of particles, in particular dust particles, from the fluid to be filtered, such as air. Various advantageous embodiments of the filter medium are possible.

According to the invention, the filter system comprises a first filter element equipped with at least one filter medium and a secondary element arranged inside the filter element, with the filter element and the secondary element extending along a longitudinal axis, a housing with a housing wall and a cover, an inlet arranged tangentially on the housing for feeding of a fluid to be filtered, in particular air, a central outlet arranged on the housing for discharging the filtered fluid, the filter medium of the filter element being folded in a zigzag shape and closed in the shape of a ring and having a plurality of outer edges extending along the longitudinal axis and the outer edges on the opposite side of the Filter medium has opposite inner edges of folds, the outer edges and the inner edges of the folds lying on the lateral surfaces of a cylinder, wherein the filter medium of the secondary element is folded in a zigzag shape and is closed in the form of a ring and one Having a plurality of outer edges extending along the longitudinal axis and inner edges of folds opposite the outer edges on the opposite side of the filter medium, the outer edges of the folds lying on a lateral surface of a cylinder and the inner edges lying on the lateral surface of a cone, the fold height being reduced on the outlet side and wherein the filter medium of the secondary element has a flow resistance variable along the longitudinal axis.

In a preferred embodiment, the cover of the housing can be arranged on an end face of the housing and thus be constructed concentrically around the longitudinal axis, while the inlet can be arranged in the housing wall or the cover. The outlet can also be arranged concentrically to the longitudinal axis in order to be able to install the filter system in a particularly space-saving manner.

In an embodiment according to the invention as a so-called pleated, i.e. zigzag, folded filter medium, in particular star-folded filter medium, the fluid to be filtered can flow against the outer edges of the filter medium, so that the deposited dirt particles remain on the outside of the filter medium, but other flow conditions can also be advantageous. With a star-pleated filter medium, this is self-contained, e.g. B. with an annular or angular cross-section. In this way, the filter medium can also be flowed through from the outside to the inside or vice versa.

On the one hand, the outer edges of the folds can lie on a lateral surface of a cylinder and the inner edges can lie at least partially on the lateral surface of a cone. This has the advantage that the outer design of the filter element and thus of the filter system can be cylindrical, which can be useful for a number of applications where space is limited. The cylindrical lateral surface is characterized in that it has the same radius along the longitudinal axis, ie straight walls. The cone-shaped lateral surface, on the other hand, shows a change in radius along the longitudinal axis, i.e. a conical shape, with an opening angle of the cone in the embodiment according to the invention being in the range from 1° to typically 10°, preferably between 1° and 5°.

Folding of the filter medium in which at least one edge of the folds lies on the lateral surface of a cone is referred to below as a semi-conical fold.

In general, a cylindrical design of a filter system is very common and useful for installation in the installation space of internal combustion engines, but the external dimensions can also have angular or flat shapes, which also result in angular and/or flat lateral surfaces for the outer edges of folds in the filter medium can have.

In the case of circular air filters, so-called star-pleated filter elements with straight folds in a cylindrical or conical design are known from the prior art. Secondary elements, i.e. filter elements that are used inside the actual filter elements to protect the clean air area from contamination when changing filter elements, covered with fleece material, also in cylindrical or conical design, belong to the prior art. The use of a semi-conical fold as described above is particularly advantageous in two-stage air filters with cyclone separators, with or without a downstream secondary element, in a cylindrical or conical design, since particularly effective particle separation is possible with a particularly small installation space requirement.

Further advantages lie in better air guidance with possible larger cross sections, for example in the area of the clean air outlet, which means that the pressure loss of the entire air filter can be reduced. In addition, the specific dust load, i.e. the dust mass per filter area on the actual filter element, is better, which leads to a longer service life of the filter element with the same installation space.

A cyclone separator is provided in the area of the inlet of the filter system (two-stage air filter system) and a dirt outlet is provided on the housing or on the cover. This cyclone separator consists of a guide geometry that causes the fluid to be filtered to rotate. This rotation concentrates the dirt in the area of the housing wall and discharges it at a suitable point via a dirt outlet. By pre-separating most of the dirt from the air to be filtered, the service life of the actual filter element can be significantly extended.

In an advantageous embodiment, a fold height of the folds can expediently change continuously along the longitudinal axis. In this way, an effective presentation of the semi-conically folded design of a filter medium according to the invention is made possible in a particularly favorable manner. Furthermore, the flow resistance along the longitudinal axis can be continuously adjusted in order to favorably influence the separation rate of the particles.

In the filter system according to the invention, one of two filter elements is arranged inside the other of the filter elements. Such a two-stage filter system is particularly advantageous for use under demanding conditions such as large amounts of dust, for example in construction and agricultural machinery, since the inner filter element can effectively protect the clean air outlet from contamination when the outer filter element is changed.

According to the invention, a secondary element is arranged inside the filter element. The secondary element, which may consist of a supporting structure covered with a permeable filter medium, e.g a fleece, has the task of keeping the outlet of the filter system closed when the filter element is replaced, so that no dirt can penetrate this area while the filter element is being cleaned or replaced. Such a filter system is particularly advantageous for use under demanding conditions such as large amounts of dust, for example in construction and agricultural machinery, since the secondary element can effectively protect the clean air outlet from contamination when the outer filter element is changed. In a preferred embodiment, the secondary element can be connected to the housing via a screw connection and can be provided with a seal relative to the housing.

A support tube can advantageously be arranged in the interior of the secondary element. An inherently unstable filter element as well as a secondary element can be reinforced by the support tube in order to retain the original shape against both flow pressure and mechanical shocks during operation of an internal combustion engine and continue to reliably perform the intended function.

In one embodiment, the secondary element can be connected to the housing and can remain in the housing when the filter element is changed. As a result, the clean air outlet is particularly effectively protected against contamination when the filter element is changed. The fixed attachment of the secondary element to the housing can also promote the production of a particularly cost-effective embodiment of the filter system.

if the secondary element is not permanently connected to the housing, but is designed to be exchangeable, then an end plate of the secondary element can be designed as a handle for better assembly/disassembly.

At least one of the one or more filter elements can expediently be arranged in an exchangeable manner in the housing of the filter system. Especially when using a filter system under particularly harsh environmental conditions, filter elements can become so loaded with dust that they have to be replaced at relatively short intervals and of course the entire filter system does not have to be replaced every time, which would lead to considerable additional expenditure on operating costs.

Using a filter system as an air filter, in particular as an air filter of an internal combustion engine, can be advantageous.

In a favorable embodiment, the filter medium can consist, for example, of a filter bellows folded in a zigzag shape (pleated) and be designed closed in the form of a ring. The folds can be made, for example, by knife folds, for longer filter bodies, or rotary folds. The filter bellows can consist, for example, of paper or of cellulose or of a mixed fiber made of plastic and cellulose. The filter bellows can also have a smooth surface, be rolled and/or have a surface designed in various embossing forms for stiffening and/or creating cavities for dust deposits. The filter bellows can have a coating and/or impregnation to repel moisture. Alternatively, it can also be coated with so-called nanofibers. The filter body may be further structurally stiffened with a filament wrap. A thread is wound around the circumference of the filter body. Several thread windings can also be provided at different axial heights of the filter body. The use of these materials as a filter medium represents a very economical way of realizing such a filter element. At the same time, the shape design described offers a stable arrangement, so that a self-supporting construction of the filter body and thus favorable assembly properties are provided.

Brief description of the drawings

Further advantages result from the following description of the drawing. Exemplary embodiments of the invention are shown in the drawings. The drawings, the description and the claims contain numerous features in combination. The person skilled in the art will expediently also consider the features individually and combine them into further meaningful combinations.

Examples show:

1

a perspective view of a two-stage filter system with a cyclone separator according to an embodiment of the invention;

2

a longitudinal section through a helpful for understanding the invention filter element with internal secondary element, wherein the cone shapes of the semi-conical folds in the longitudinal axis in the same direction;

3

a longitudinal section through a helpful for understanding the invention filter element with internal secondary element, wherein the cone shapes of the semi-conical folds in the longitudinal axis in the opposite direction;

4

a longitudinal section through a filter system according to an embodiment of the invention, in which the filter medium of the filter element has a straight fold and an internal secondary element has a semi-conical fold;

figure 5

a longitudinal section through a helpful for understanding the invention filter system, in which the filter medium of the filter element and the internal secondary element have a semi-conical fold pointing in the longitudinal axis in the same direction;

6

a longitudinal section through a helpful for understanding the invention filter system, in which the filter medium of the filter element has a semi-conical fold and the filter medium of the secondary element consists of a fleece;

7

a filter element according to an embodiment of the invention with semi-conical pleating of the filter medium; and

8

a filter medium according to an embodiment of the invention with semi-conical folds of the filter medium.

Embodiments of the invention

In the figures, the same or similar components are denoted by the same reference symbols. The figures only show examples and are not to be understood as limiting.

figure 1 shows a perspective view of a two-stage filter system 100 with a cyclone separator 36 according to an embodiment of the invention with a tangential inlet 102, a central outlet 104 on a housing front side and a dirt outlet 106 on the bottom. A round filter design is shown, which consists of a housing 108 which has a housing wall 112 and is closed with a cover 110, for example with a screw or bayonet lock. When used as an air filter system, dust-laden air flows into the inlet 102, which is arranged tangentially to the internally installed air filter element, so that the air inside the housing 108 is set into a rotational movement by an inflow protection on the filter element. Filter element and flow protection are not shown in the drawing. Due to the cyclone effect brought about by the rotational movement of the air, centrifugal forces act on the dust particles in the flowing air, so that they are partially separated on the housing wall and can flow out of the filter system 100 via the dirt outlet 106 . This puts less strain on the filter element and increases the service life of the filter element. The cleaned air can be discharged from the housing 108 via the central outlet 104 .

Such filter systems, as in figure 1 shown, are usually used as air and / or particle filters in particular for internal combustion engines in the field of construction and agricultural machinery. They are characterized by great robustness and have a short service life due to the high filter load. A filter system 100 with a loaded filter element must tolerate an increase in weight of 10 kg or more.

figure 2 shows a longitudinal section through a filter element 10 with an internal secondary element 12, 28, which is helpful for understanding the invention, the cone shapes of the semi-conical folds of the filter medium 56 pointing in the longitudinal axis L in the same direction. In figure 2 a filter element 10 is shown, in the interior 50 of which a second filter element 12 is arranged, which is used, for example, as a secondary element 28 . The filter element 10 is closed at both ends with a first and a second end plate 16, 18, which give the filter element 10 stability and ensure sealing when installed in a housing 108 of a filter system 100. The secondary element 28 is closed off at the top with an end disk 30 and is open at the bottom so that the filtered fluid can escape. The filter element 10 is arranged in such a way that the inner edges 72 of the folds of the filter medium 56 lie on the lateral surface of a cylinder, while the outer edges 70 lie on the lateral surface of a cone, with the filter element 12 being arranged in such a way that the inner edges 72 of the folds 62 of the Filter medium 56 lie on the lateral surface of a cone, while the outer edges 70 lie on the lateral surface of a cylinder. In this way, both filter elements 10, 12 can be arranged as close together as possible, while the interior of the filter elements 10, 12 opens wide in the opposite direction to the longitudinal axis L, in the outlet direction, thus ensuring a good flow for the filtered fluid.

figure 3 shows a longitudinal section through a filter element 10 with an internal secondary element 12, 28, which is helpful for understanding the invention, the cone shapes of the semi-conical folds of the filter medium 56 pointing in the longitudinal axis L in the opposite direction. In figure 3 a filter element 10 is shown, in the interior 50 of which a second filter element 12 is arranged, which is used, for example, as a secondary element 28 . The filter element 10 is closed at both ends with a first and a second end plate 16, 18, which give the filter element 10 stability and ensure sealing when installed in a housing 108 of a filter system 100. The secondary element 28 is closed off at the top with an end disk 30 and is open at the bottom so that the filtered fluid can escape. The filter element 10 is arranged in such a way that the inner edges 72 of the folds 62 of the filter medium 56 lie on the lateral surface of a cylinder, while the outer edges 70 lie on the lateral surface of a cone, with the filter element 12 being arranged in such a way that the inner edges 72 of the folds 62 of the filter medium 56 lie on the lateral surface of a cylinder, while the outer edges 70 lie on the lateral surface of a cone. In this way, the interior 50 presents a cylindrical open Area is, so that the filter element 12 can be plugged, for example, on a cylindrical support tube.

In figure 4 1 is a longitudinal section through a filter system 100 according to an exemplary embodiment of the invention, in which the filter medium 56 of the filter element 10 has a straight fold and an internal secondary element 28 as the second filter element 12 has a semi-conical fold. The housing 108 of the filter system 100, which comprises a housing wall 112, is closed with a cover 110 at one end. A filter element 10, which includes a filter medium 56 arranged concentrically to a longitudinal axis L, is closed at two opposite end faces 15, 17 with a first and a second end plate 16, 18, which can be made of polyurethane foam or an elastomer, for example. The filter medium 56 can, for example, be folded in a zigzag shape, be closed in the form of a ring and consist, for example, of paper or of cellulose or of a mixed fiber made of plastic and cellulose.

The second end plate 18 has support knobs 20 which, when installed in the receiving housing 108 , resting against an inner cover contour 114 of the cover 110 , are supported on the housing 108 both axially and radially. A radial seal 26 is attached to the first end plate 16 on the opposite end face 15 of the filter element 10, with the aid of which the filter element 10 is supported radially with a radial seal 26 via the sealing contour 116 on the housing 108 and seals the unfiltered air space from the filtered air space. The filter element 10 is thus braced against the housing 108 both axially and doubly radially.

Dust-laden air can flow in through the inlet 102 in the direction of the arrow 40 , which in this case is shown as a tangential inlet and enables cyclone operation due to the rotational movement of the air brought about with the aid of a cyclone separator 36 . Dust particles, partially pre-separated by the rotational movement, can deposit on the inner housing wall and be emptied downwards through the dirt outlet 106 when the filter housing 108 is installed in a horizontal position by gravity out of the filter system 100 . During operation, after partial separation of the dust particles, the air flows through the filter medium 56 in the direction of the arrow 42, 44 into the interior 50 of the filter element. Depending on the filter medium, dust particles get stuck in the filter medium 56 from a certain size. Depending on the amount of dust that has entered, the filter element 10 must therefore be replaced after a certain downtime.

The filtered air flows out in the direction of arrow 46 via the outlet 104 . In the interior 50 of the filter element 10, i.e. in the clean air area, the secondary element 28 is attached, which essentially consists of a supporting structure, the support tube 14 and a relatively permeable filter medium 56, which is figure 4 is shown folded in a semiconical manner, and when the filter element 10 is replaced in the housing 108 to protect the further air duct, for example an internal combustion engine, against penetrating dust particles and other objects. The secondary element 28 is attached to the outlet-side part of the housing 108 and sealed with a radial seal.

The cone of the semi-conical folds of the secondary element 28 is arranged in such a way that it opens in the opposite direction to the longitudinal direction L, ie in the outlet direction, in order to represent the lowest possible flow resistance for the filtered fluid. The inner edges 72 of the folds 62 of the secondary element 28 lie on the lateral surface of a cone, while the outer edges 70 lie on the lateral surface of a cylinder. The filter medium 56 of the filter element 10 has a straight fold, in which the inner and outer edges 72, 70 of the folds 62 lie on the lateral surfaces of a cylinder.

The advantage of the in figure 4 illustrated embodiment is that the secondary element 28 can be designed cylindrical in its outer contour and thus can be plugged onto a support tube 14 fixed to the housing. At the same time, a secondary element 28, equipped with fleece, can optionally be provided in the same housing for low requirements with regard to the degree of separation and pressure loss.

Due to the semi-conical design, a larger filter surface can be provided on the secondary element 28 by a possibly smaller fold division, in comparison to a straight fold of the same fold height 74 (on the outlet side). The larger filter surface also applies to the comparison with the fleece medium, which cannot be folded at all at this low height and thus only forms the cylindrical surface of the outer contour.

The large filter area together with the still large outlet cross-section on the outlet side then offers the advantage of a very low pressure loss.

figure 5 12 shows a longitudinal section through a filter system 100 useful for understanding the invention, in which the filter medium 56 of the filter element 10 and the inner secondary element 28 have a semi-conical fold pointing in the longitudinal axis L in the same direction.

The cone of the semi-conical folds of the secondary element 28 is arranged in such a way that it opens in the opposite direction to the longitudinal direction L, ie in the outlet direction, in order to represent the lowest possible flow resistance for the filtered fluid. Both the inner edges 72 of the folds 62 of the secondary element 28 and the outer edges 70 lie on the lateral surfaces of two cone shapes. The secondary element 28 is mounted on a support tube 14 which is also conical. The filter medium 56 of the filter element 10 also has a semi-conical fold, in which the inner edges 72 on the lateral surface of a cone and the Outer edges 70 of the folds 62 lie on the lateral surfaces of a cylinder. Since the cone shape of the secondary element 28, like the cone shape of the filter element 10, is open in the opposite direction to the longitudinal axis L, i.e. in the outlet direction, the tightest possible packing of the filter element 10 and the secondary element 28 is possible, so that the interior 50 of the filter element 10 extends as far as possible towards the outlet 104 can open towards and thus opposes the lowest possible flow resistance to the filtered fluid.

The advantage of the in figure 5 illustrated embodiment is that the cone shape is now even more pronounced by this arrangement in the interior 50, which further optimizes the flow control and thus the pressure loss.

Due to the semi-conical folds of the filter medium 56, a larger filter area can be provided on the secondary element 28 due to a possible smaller fold division compared to straight folds of the same fold height 74 (on the outlet side). The larger filter surface also applies to the comparison with the fleece medium, which cannot be folded at all at this low height and thus only forms the cylindrical surface of the outer contour.

The large filter surface together with the still large outlet cross-section on the seal side then offers the advantage of a very low pressure loss.

The filter element 10 equipped with semi-conical folds now offers the possibility, on the one hand, of opening the outlet cross-section further on the outlet side by reducing the fold height 74 on this side (positive for the pressure loss), and on the other hand of not having any loss of filter area, because the Pleat height 74 is twice as large on the closed side. In addition, the specific dust load (dust mass per filter area) of the filter element is improved because large amounts of dust can be absorbed through the large interstices in the pockets of the folds 62 .

In figure 6 1 is a longitudinal section through a filter system 100 helpful for understanding the invention, in which the filter medium 56 of the filter element 10 has a semi-conical fold and the filter medium 56 of the secondary element 28 consists of a fleece. The filter medium 56 of the filter element 10 has a semi-conical fold in which the inner edges 72 lie on the lateral surface of a cone and the outer edges 70 of the folds 62 lie on the lateral surfaces of a cylinder. The secondary element 28 is adapted to the conical shape of the inner edges 72 of the filter element 10 and comprises a fleece that is attached to a cone-shaped support tube 14 .

The conical shape of the filter medium 56 in the form of a fleece as the secondary element 28 enables a large outlet opening and a larger filter area than a comparable straight secondary element 28 made of a fleece and having a smaller diameter.

The secondary element 28 thus does not require its own central tube made of plastic or metal and can therefore be produced more cost-effectively. A plastic ring 32 to which the fleece is attached and to which an O-ring seal 34 is provided is useful for stiffening.

Alternatively, instead of a fleece, a simple star-folded filter bellows in a conical shape adapted to the inside of the filter element 10 can be used, which preferably has a constant fold height that is low compared to the filter element 10, for example less than or equal to 10 mm, preferably less than or equal to 5 mm .

The filter element 10 equipped with semi-conical folds now offers the possibility, on the one hand, of opening the outlet cross-section further on the outlet side by reducing the fold height 74 on this side (positive for the pressure loss), and on the other hand of not having any loss of filter area, because the Pleat height 74 is twice as large on the closed side. In addition, the specific dust load (dust mass per filter area) of the filter element is improved because large amounts of dust can be absorbed through the large interstices in the pockets of the folds 62 .

figure 7 shows a filter element 10 according to an embodiment of the invention with semi-conical folds of the filter medium 56. The filter element 10 is closed at both ends with a first and a second end plate 16, 18, which are used for the stability of the filter element 10 and for sealing during installation in the housing 108 of a filter system 100 are used. The filter medium 56 has a semi-conical fold such that the outer edges 70 of the folds 62 lie on the lateral surface of a cylinder and the inner edges 72 lie on the lateral surface of a cone.

figure 8 further shows a filter medium 56 according to an embodiment of the invention with semi-conical folds of the filter medium 56. The outer edges 70 of the folds 62 of the filter medium 56 lie on the lateral surface of a cylinder and the inner edges 72 lie on the lateral surface of a cone. One recognizes in figure 8 that the folds 62 become denser in the longitudinal direction L, while the fold height 74 decreases continuously in the longitudinal direction L, so that the cone consequently opens in the longitudinal direction L. The filter medium 56 thus has a flow resistance that is variable along the longitudinal axis L, whereby it increases on the outside in the exemplary embodiment shown, while it decreases along the longitudinal axis L on the inside due to the opening inner cross section.

Claims (7)

1. Filter system (100), comprising

   - a first filter element (10) equipped with at least one filter medium (56),

   - and a secondary element (28) disposed inside (50) the filter element (10), the filter element (10) and the secondary element extending along a longitudinal axis (L),

   - a housing (108) with housing wall (112) and a cover (110),

   - an inlet (102) tangentially disposed at the housing (108) for supplying a fluid to be filtered, in particular air,

   - a cyclone separator (36) which causes a rotational movement of the supplied fluid to be filtered, in particular the air,

   - a dirt outlet (106) through which dust particles partially pre-separated by the rotational movement and deposited on the inner housing wall are discharged,

   - a central outlet (104) disposed at the housing (108) for discharging the filtered fluid,

   wherein the filter medium (56) of the filter element (10) is zigzag-folded and realized in an annularly closed design and features a plurality of outer edges (70) extending along the longitudinal axis (L) and inner edges (72) of folds (62) opposing the outer edges (70) on the opposing side of the filter medium (56), wherein the outer edges (70) and the inner edges (72) of the folds (62) rest on the circumferential surfaces of a cylinder, wherein the filter medium (56) of the secondary element (28) is zigzag-folded and realized as an annularly closed design and features a plurality of outer edges (70) extending along the longitudinal axis (L) and inner edges (72) of folds (62) opposing the outer edges (70) on the opposing side of the filter medium (56), wherein the outer edges (70) of the folds (62) rest on a circumferential surface of a cylinder and the inner edges (72) rest on the circumferential surface of a cone, wherein the fold height on the outlet side is reduced, and wherein the filter medium (56) of the secondary element (28) features a variable flow resistance along the longitudinal axis (L).
2. Filter system according to claim 1, wherein a fold height (74) of the folds (62) of the secondary element (28) changes continuously along the longitudinal axis (L).
3. Filter system according to one of the above claims, wherein a support tube (14) is disposed inside the secondary element (28).
4. Filter system according to one of the above claims, wherein a filter medium (56) of the secondary element (28) consists of a non-woven fabric.
5. Filter system according to one of the above claims, wherein the secondary element (28) is connected to the housing (108) and remains in the housing (108) when changing the filter element (10).
6. Filter system according to one of the above claims, wherein at least one of the filter elements (10, 12) is replaceably disposed in the housing (108) of the filter system (100).
7. Use of a filter system (10) according to one of the above claims as air filter, in particular as air filter of an internal combustion engine.

Images